Perovskite-based fuel cell electrode and membrane

ABSTRACT

The present invention provides a material suitable for use in a solid oxide fuel cell, wherein the material is of an, optionally doped, double perovskite oxide material having the general formula (I): (Ln a X b ) e (Z 1   c Z 2   d ) f O g  (I) wherein Ln is selected from Y, La and a Lanthanide series element, or a combination of these and X also represents an element occupying the A site of a perovskite oxide and is selected from Sr, Ca and Ba, and Z 1  and Z 2  represent different elements occupying the B site of a perovskite oxide and are selected from Cr, Mn, Mg and Fe, and wherein a has a value from 0 to 1, preferably 0.7 to 1.0, b has a value of from 1 to 0, preferably 0.3 to 0, and each of c and d has a value of from 0.25 to 0.75, provided that a+b has a value of 1, and c+d, has a value of 1, and wherein e has a value of from 0.8 to 1, wherein f has a value of from 0.8 to 1, and g has a value of from 2.5 to 3.2. Also provided are SOFCs having an electrode or functional layer of a material or containing a material of the invention, as well as mixed ionic/electronic conducting membranes suitable for use in a syngas reactor or oxygen separator, comprising a layer of a double perovskite material of the invention, and a method of oxidising a fuel in an SOFC having an anode of a double perovskite material of the invention.

FIELD OF THE INVENTION

The present invention relates to fuel cell electrodes, and moreparticularly to electrodes suitable for use in solid oxide fuel cells(SOFCs).

BACKGROUND OF THE INVENTION

There is a great need for and interest in more efficient means ofconverting chemical energy into electrical energy, which has createdgreat interest in fuel cells. The practical commercial development ofthese has, though, been held back by numerous practical problems. TheSOFC is a particularly attractive system, which can utilize hydrocarbonfuels such as methane with internal reforming of the fuel, and achieverelatively high efficiencies. Significant problems remain, though, inrelation to inter alia the design of the anode used.

Many different materials have been tried in the search for improvedanode performance, including materials such as Ni/YSZ (YSZ=Y₂O₃/ZrO₂)which has relatively good performance but suffers from the substantialdisadvantage of relatively short working life due to the formation ofcarbon deposits when using hydrocarbon fuels, susceptibility topoisoning with sulphur which is frequently encountered in hydrocarbonfuels and an intolerance to repeated reduction/oxidation cycling, as wasencountered in small systems such as CHP units or APUs for automotiveapplications. It has also been proposed to use LaCrO₃ (P. Vernoux et alJ. Electrochem. Soc. 145 3487-3492 (1998)), and more recently it hasbeen proposed to utilize LaCrO₃ which has been doped with variouselements in order to improve its performance (J. Liu et alElectrochemical and Solid-State Letters 5 A122-A125 (2002)).Nevertheless even such doped LaCrO₃ has relatively limitedelectrochemical performance and effective working life.

It is an object of the present invention to avoid or minimize one ormore of the above mentioned disadvantages.

It has now, by consideration of a novel approach, been found that byusing a double perovskite material based on LaCrO₃ instead of a dopedLaCrO₃, it is possible to achieve electrical and catalytic propertiescomparable with prior art anode materials such as Ni based anodes (thatis with over-potential losses which can be less than 100 mV at currentdensities of 400 mAcm⁻²) without the need for using metallic currentconducting components, normally nickel and without significant carbonformation and deposition when using hydrocarbon fuels. Unlike previouslytried doped LaCrO₃ in which a small number of the La and/or Cr atoms inLaCrO₃, typically 5 to 10%, or at most 20%, are replaced by differentatoms, resulting in a doped form of a “single” perovskite, in a doubleperovskite material the B sites of the perovskite crystal latticestructure, normally occupied substantially wholly by Cr, are occupied bysimilar amounts of two different elements. It should be emphasised thatthe term double perovskite is used here to emphasise double occupationB-site, and not necessarily to imply any structural order that manifestsitself as two different structural B-sites existing in the lattice.

SUMMARY OF THE INVENTION

Thus in one aspect the present invention provides a novel materialsuitable for use in a solid oxide fuel cell, especially in an anodethereof, wherein the material is of an, optionally doped, doubleperovskite oxide material having the general formula I:(Ln_(a)X_(b))_(e)(Z¹ _(c)Z² _(d))_(f)O_(g)  (I)wherein Ln is selected from Y, La and a Lanthanide series element, or acombination of these and X also represents an element occupying the Asite of a perovskite oxide and is selected from Sr, Ca and Ba, and Z¹and Z² represent different elements occupying the B site of a perovskiteoxide and are selected from Cr, Mn, Mg and Fe, and wherein a has a valuefrom 0 to 1, preferably, 0.7 to 1.0, b has a value of from 1 to 0,preferably 0.3 to 0, and each of c and d has a value of from 0.25 to0.75, provided that a+b has a value of 1, and c+d, has a value of 1, andwherein e has a value of from 0.8 to 1, wherein f has a value of from0.8 to 1, and g has a value of from 2.5 to 3.2.

Thus it will be appreciated that although the Z¹ and Z², elements arepreferably present in substantially equivalent amounts, they cannevertheless depart to some extent from exactly equivalent amounts. Alsoit is possible for the A site in the perovskite material (occupied by Lnand X), and/or the B site in the perovskite material (occupied by Z¹ andZ²), to be somewhat depleted (e<1 and f<1) without disrupting thecrystal structure thereof and significantly degrading the properties ofthe anode. Similarly, although g normally has a value of 3, some crystaldefects in relation to the O sites are also acceptable. Moreparticularly some O site deficiency (g<3) is acceptable and indeed maybe useful in that it allows for O atom mobility between different Osites within the crystal lattice of the material. A limited degree ofexcess O atoms (g>3) is also acceptable in at least some circumstances.

The novel double perovskite materials provided by the present inventioncan be used in the production of anodes for use in SOFCs and which havesignificantly improved electrochemical performance, electrical andcatalytic properties when compared with anodes of previously knownmaterials, when used with methane fuel, whilst avoiding the particularproblems and disadvantages of previously proposed electrodes such asNi—YSZ. More particularly it has been found that they are chemicallyredox stable, which may significantly decrease the volume instabilityduring redox cycling that causes degradation.

Whilst the novel materials of the present invention are particularlyvaluable for use as anodes in SOFCs, they also have other applicationsin SOFCs including as one or more of: anode functional layer, cathodefunctional layer, cathode, and interconnector.

As used herein “functional layer” indicates a thin electroactive layerprovided between the electrode current collector (anode or cathode) andthe electrolyte; or possibly between the anode or cathode currentcollector and another functional layer, for the purposes of protectingthe electrode itself from degradation (typically due to interfacialreaction), and/or enhancing catalytic activity and/or enhancing theperformance characteristics (e.g. reducing polarisation resistance).Typically such a functional layer could have a thickness of from 1 to 50μm, preferably 20 to 30 μm. The functional layer may moreover besubstantially solid or have more or less greater porosity, for example,up to 70% porosity, preferably from 30 to 60% porosity, convenientlyfrom 40 to 50% porosity.

The term interconnector indicates a component used for connectingtogether the electrodes of neighbouring cells in an assembly of aplurality of cells connected together in series. In this case thematerials are preferably made with high density (i.e. low porosity,preferably not more than 1% porosity, advantageously zero continuousporosity) in order to make them substantially gas-impermeable.

As indicated above, materials of the present invention can,surprisingly, be used as both anodes and cathodes. This makes itpossible to produce a cell with the same anode and cathode electrodeswhereby it is possible to operate the cell using either electrode ascathode and anode, and even to reverse operation of the cell whenrequired, for example, in a reversible fuel cell application (in whichenergy can be stored temporarily by applying a voltage to theelectrodes), by changing the cell connections so that the anode becomesthe cathode and vice versa.

Preferably Z¹ and Z² represent Cr and Mn, respectively.

Advantageously X represents Sr.

Although it is a particular feature of the novel materials provided bythe present invention, that they have a distinctly different nature andcomposition from the previously known doped single perovskite materials,the double perovskite materials used in accordance with the presentinvention may nevertheless also be doped to some extent i.e. any one ormore of the A and B sites which would otherwise be occupied by Ln, X, Z¹and Z², may be replaced to a limited degree by one or more suitabledopant elements in order to improve still further electrical and/orcatalytic properties.

Suitable dopants at the B sites i.e. replacing either or both of some Z¹and some Z² sites, for improving electrical conductivity include V, Fe,Cu, Co, Ti, Nb, Mo, Ru and Ni, whilst suitable dopants for improvingcatalytic activity include Pd, Ce, Ni, Ru and Mg. In general the dopantshould not occupy more than 20% of the B sites of the double perovskiteoxide. Where there is used a dopant, this is preferably present at alevel of not more than 20%, preferably from 5 to 20%.

The novel materials of the present invention may be used with variousdifferent electrolytes. In particular they have been found to becompatible and suitable for use with yttria stabilised zirconia (YSZ)electrolyte which exhibits good thermal and chemical stability. As usedherein the expression “double perovskite” indicates a material in whichthe B sites in the crystal lattice structure of said material arepopulated by comparable substantial amounts of two different elements,with not less than 25%, preferably not less than 30% of the B sitespopulated by said two different elements. Thus, although the inventionencompasses materials in which one of the elements can occupy as many as75% of the B sites, and the other as few as 25%, substantial amounts ofeach one (at least 25%, preferably at least 30%) are present, and therelative amounts (3:1 or less) are comparable—unlike in a doped materialin which the elements are in a relative ratio of at least 4:1 and often10:1 or more.

It should also be noted that the B sites could also be occupied by athird element (Z3) present in an amount of at least 30%, which iscomparable to that of the first two elements. Whilst such a materialcould be described as a triple perovskite, it should be understood to bealso encompassed within the “double perovskites” of the presentinvention. Again the term “triple” reflects composition at the B-siterather than a specific type of structural ordering.

Preferred materials provided by the present invention are those in whichin general formula I, each of c and d has a value of at least 0.4.Desirably, in general formula I, a has a value of from 0.7 to 0.9, mostpreferably from 0.72 to 0.85.

Particularly preferred Z¹ and Z² species are Cr and Mn, whilst aparticularly preferred X species is Sr.

The novel materials of the present invention may be prepared by anysuitable method known in the art. In general two or more compoundsconsisting essentially of the required metallic elements in suitableproportions, in the form of suitable oxides or salts with nitric acids,are brought together into intimate admixture with heat treatment. Oneconvenient method comprises a solid state reaction in which dry oxidesand/or carbonates (or other salts such as acetates, oxalates etc) of themetal elements are mixed together and fired at a high temperature,typically of the order of 1000 to 1400° C. Another convenient methodcomprises combustion synthesis in which a solution of salts such asnitrates of the required metal elements in suitable proportions, inaqueous ethylene glycol, from which solution water is progressivelyremoved to yield a gel which can be burnt to provide a char. Firing ofthe char at high temperature, typically above 1100° C., then yields thedouble-perovskite form of the material.

In order to make the material suitable for use as a fuel cell electrode,it is necessary for it to be in a relatively porous form which presentsa relatively large surface area for chemical interactions to take placeat. Preferably the electrode should have a porosity value of at least20%, preferably from 30 to 60% advantageously from 40 to 50%, typicallyaround 50%. In general suitably porous forms of the novel doubleperovskite material may be obtained by the addition of pore-formingagents (PFAs). The PFAs could be one or more of carbon and organicmaterials, such as PVB (polyvinyl butyral), PEG (polyethylene glycol),terpineol, ethyl cellulose etc.

The novel materials of the present invention may be used in variousforms and configurations of SOFCs. Thus they may be used as anode and/orcathode, and/or functional layer, in planar or tubular SOFC or SOFCrolls.

The novel materials of the invention are substantially compatible withvarious electrolytes used in SOFCs, including in particular doped ceriaand especially perovskites such as Sr- and Mg-doped LaGaO₃ etc, as wellas undoped ceria.

In another aspect the present invention provides a SOFC having anelectrode or functional layer of a novel material according to thepresent invention.

In a preferred aspect the present invention provides a SOFC having ananode of a novel material according to the present invention.

SOFCs using or containing an anode of the present invention may be usedto oxidise any fuel appropriate for fuel cell use either directly orafter at least partial reformation. Such fuels include hydrogen;hydrogen; a hydrocarbon fuel compound such as methane, ethane, propane,or butane; a hydrocarbon based fuel compound such as methanol orethanol; and a non-hydrocarbon hydride fuel compound such as ammonia,hydrogen sulphide; as well as mixtures of such compounds such as LPG,gasoline, diesel, biogas, biofuel, kerosene, or JP8.

Thus in another aspect the present invention provides a method ofoxidising a fuel in an SOFC, comprising the steps of:

-   a) providing an SOFC having an anode of the novel material of the    present invention; and-   b) applying a voltage to said SOFC so as to oxidize said fuel.

It is also possible in at least some cases to use the new doubleperovskite materials provided by the present invention, in fuel cellcathodes, and accordingly the present invention also extends to a fuelcell cathode comprising a substantially porous body of a doubleperovskite according to the present invention. This provides theopportunity to use the same material as both cathode and anode in a SOFCwith clear advantages with regard to compatibility issues.

In another aspect the invention provides a novel material suitable foruse in an anode in a solid oxide fuel cell, wherein the material is ofan, optionally doped, double perovskite oxide material having thegeneral formula I:(Ln_(a)X_(b))_(e)(Z¹ _(c)Z² _(d))O₃  (I)wherein Ln is selected from Y, La and a Lanthanide series element, or acombination of these and X represents an optional second elementoccupying the A site of a perovskite oxide and is selected from Sr, Caand Ba, and Z¹ and Z² represent different elements occupying the B siteof a perovskite oxide and are selected from Cr, Mn and Fe, and wherein ahas a value from 0.7 to 1.0, b has a value of from 0.3 to 0, and each ofc and d has a value of from 0.25 to 0.75, provided that a+b has a valueof 1, and c+d, has a value of not less than 0.8, and wherein e has avalue of from 0.8 to 1.

The novel double perovskite materials of the present invention may alsobe used in mixed-conducting ceramic membranes as a syngas reactormembrane or as a protective layer on the natural gas side of a syngasreactor membrane layer of another material (which typically comprises adense layer of lanthanum strontium-iron-cobalt oxide) or a relatedcomposition. Such ceramic membranes are useful for partial oxidation ofnatural gas into synthesis gas, often referred to as syngas. Syngas canbe used to make liquid diesel and other transportation fuels, as well aschemicals for the petrochemical, rubber, plastics, and fertilizerindustries. Hydrogen can also be separated from the gas and used as anenergy source or by a refinery to produce cleaner, higher-performancegasoline. The unique approach of the MIEC (mixed ionic/electronicconducting) membrane technology allows the integration of oxygenseparation, steam- and CO₂-reforming, and partial oxidation of methaneinto a single process. By eliminating the need for a separateoxygen-production plant, the technology substantially reduces the energyand capital cost associated with conventional syngas production. TheMIEC membrane technology can also help to reduce NO_(x) emissions byusing nitrogen oxides as an oxygen source. In addition, the energycontained in the oxygen-depleted air stream can be recovered through thegeneration of power and steam.

Thus in a further aspect the present invention provides a mixedionic/electronic conducting membrane comprising a layer of a noveldouble perovskite material according to the present invention. Typicallysaid layer comprises a protective layer on at least one side of a mixedionic/electronic conducting ceramic membrane, and especially onesuitable for use in a syngas reactor. Such a protective layer wouldgenerally have a thickness of from 1 to 200 μm, preferably from 20 to 70μm. Where the membrane consists essentially of a novel double perovskitematerial according to the present invention, the membrane wouldgenerally have a thickness of from 10 to 500 μm, preferably from 20 to100 μm. Such mixed ionic/electronic conducting membranes are alsosuitable for use in separating oxygen from air for various purposes,e.g. for the production of (substantially pure) oxygen gas, or for usedirectly in reactions with other materials (e.g. with methane in syngasproduction). It will be appreciated that in such membrane applications,the perovskite material should be made with high density (i.e. lowporosity, preferably not more than 1% porosity, advantageously zerocontinuous porosity) in order to make them substantiallygas-impermeable. Such membranes may be self supporting or supported on aporous metal or ceramic or metal/ceramic composite, support.

BRIEF DESCRIPTION OF THE DRAWINGS

Further preferred features and advantages of the invention will appearfrom the following detailed examples given by way of illustration, anddrawings in which:

FIGS. 1, 3 and 4 are graphs showing measurement of various electricalproperties obtained using anodes of the invention;

FIG. 2 is a schematic drawing of the principal parts of anelectrochemical cell of the invention in sectional elevation; and

FIG. 2A is an underside plan view of the cell of FIG. 2; and

FIG. 5 is a schematic sectional view of an SOFC.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 Preparationof Double Perovskite Material

La₂O₃ (4.8873 g), SrCO₃ (1.4763 g), Cr₂O₃ (1.5199 g) and MnO₂ (1.7388 g)in powder form were mixed together in an agate mortar. The mixed powderswere transferred into a zirconia container, with addition some acetoneor ethanol and ball-milled for 15 minutes twice, and then left for 10hours in a fume cupboard to evaporate the organic component.

The dried powders were then subjected to a series of high temperaturefiring and intermediate grinding cycles using a muffle furnace, asfollows:

-   -   1. 12 hours at 1400° C.    -   2. 20 mins grinding    -   3. 12 hours at 1400° C.    -   4. repeat 2&3 once

The above procedure yielded 9 g of a double perovskite in the form of apowder having the composition La_(0.75)Sr_(0.25)Cr_(0.5)Mn_(0.5)O₃(LSCM). This phase exhibits a rhombohedral unit cell, a=5.4562(3) Å,α=60.440 (9)°.

EXAMPLE 2 Preparation of Double Perovskite Material

La₂O₃ (4.8873 g), SrCO₃ (1.4763 g) and MnCO₃ (2.299 g) were dissolved indilute nitric acid (40 mls 4 N) and heated to 80° C. with stirring untila solution is obtained. Then 8.0028 g Cr(NO₃)₃.9H₂O was dissolved intothe solution. 25 ml pure ethyl glycol was then added into the mixednitrate solution and stirred at 80° C. for 2 hours. The obtained gel wastransferred into a porcelain container and heated on a hot plate untilfiring into char. The char was further heated at 1100 to 1400° C. toobtain the perovskite oxide having the compositionLa_(0.75)Sr_(0.25)Cr_(0.5)Mn_(0.5)O₃.

EXAMPLE 3 Manufacture of Anode

The double perovskite material of Example 1 (0.45 g) was mixed with 0.05g carbon (graphite) and 5 ml ethanol. The mixture was ground in an agatemortar or ball-milled for 30 minutes to form a slurry. The slurry waspainted or screen-printed onto an 8 mol % Y₂O₃ stabilised zirconia (YSZ)2 cm diameter disc with 2 mm thickness. The anode supported on the YSZelectrolyte was then fired from room temperature heating up at 5° C./minup to 1100° C., held at 1100° C. for 4 hours, and then cooled down toroom temperature at 5° C./min. The thickness of the anode so obtained isabout 30-100 μm with an area of 1 cm². A small amount of gold paste waspartially coated onto the anode (ca. 50% coverage) and fired at 900° C.for 30 minutes using a 5° C./min heating up and cooling down rate in therange from 300° C. to 900° C., to ensure better electronic contact fortesting purposes. Platinum paste (Engelhard Clal 6082) was painted ontothe opposite side of the YSZ pellet and fired at 900° C. for 30 minuteswith 5° C./min heating up and cooling down rates above 300° C. toprovide a counter electrode (or cathode) and reference electrode,approximately 50 μm thick.

EXAMPLE 4 Use of Anode

The anode obtained in Example 3 was mounted in a solid oxide fuel cellconfigured in the form of a 0.2 mm thick layer of YSZ electrolyte.La_(0.8)Sr_(0.2)MnO₃ (LSM) was coated onto the other side of the YSZsheet to provide a cathode. The slurry composition used in theproduction of the cathode was 0.45 g La_(0.8)Sr_(0.2)MnO₃, 0.05 ggraphite and 5 ml ethanol. A thin layer platinum paste (see Example 3)was coated onto the LSM and fired at 900° C. for 30 minutes with 5°C./min heating up and cooling down rates above 300° C. to provide acathode current collector.

FIG. 1 shows the performance of the cell using the double perovskiteoxide La_(0.75)Sr_(0.25)Cr_(0.5)Mn_(0.5)O₃ anode of Example 3, whensupplied with various different fuels: wet H₂, wet 5% H₂ or wet and pureCH₄, at 900° C. The cell had a 0.2 mm electrolyte, and data are shownfor the following fuels, wet H₂; dash, wet 5% H₂; dash-dot, wet CH₄ anddash-dot-dot CH₄ at 1173K. While the open circuit voltages (OCVs) forwet 5% H₂ and wet H₂ were close to the value predicted by the Nerstequation, 0.95 and 1.09V at 900° C., the OCV for wet and unhumidifiedcylinder CH₄ was 0.87 and 086V respectively, which is slightly lowerthan that for wet 5% H₂. The maximum power densities were higher for wetH₂ than wet 5% H₂, with values of 0.34 W cm⁻² and 0.17 W cm⁻²respectively. The maximum power density for wet methane was about 0.1 Wcm⁻² at 0.53V, which is slightly lower than that for wet 5% H₂.

EXAMPLE 5 Properties of Anode

The properties of the anode of Example 3 were examined by means of athree-electrode configuration test cell illustrated schematically inFIGS. 2 and 2A, which show a disc shaped electrolyte 1 with an annularanode 2 on one face 3 and an annular cathode 4 and a central disc-formreference electrode 5 on the opposite face 6. The electrolyte wassintered 8 mol % Y₂O₃ stabilized ZrO₂ (YSZ) pellet with 2 mm thicknessand 20 mm diameter. An anode with a thickness of about 50 μm wasdeposited onto the YSZ electrolyte using an ethanol-based slurry andfiring typically at 1000 to 1300° C. Pt paste (as previously described)was painted onto the other side of YSZ as counter or cathode, andreference electrodes. The anode over-potential with wet H₂ is shown inFIG. 3. It was found that the anode resistance decreases underpolarization which is closer to the real operation conditions than OCV.The polarization resistance is less than 0.3 Ω/cm² at a current density300 mA/cm². With further optimization, this performance could readily beimproved even more. FIG. 3 shows the potential and current change at925° C. under operation using wet CH₄ as fuel at 0.4V bias using onlyLa_(0.75)Sr_(0.25)Cr_(0.5)Mn_(0.5)O₃ as the anode. No significantperformance degradation was observed during four hours operationalthough a trace amount of carbon was observed after the fuel cellperformance in wet CH₄ and cooling down in the same atmosphere.

EXAMPLE 6 Preparation and use of Modified Anode

In some cells, a thin film interface of Ce_(0.8)Gd_(0.2)O₂ (CGO),prepared by a sol-gel process, was applied between the YSZ electrode andthe anode. Anode polarisation resistance was further decreased with sucha thin layer (5 μm) of CGO deposited between the YSZ electrolyte andLSCM anode as shown in FIG. 4 which illustrates a comparison ofelectrode impedance spectra for LSCM/CGO anodes. Spectra were measuredat 925° C. in 4.9% H+2.3% H O+92.8% Ar(Y) and 97% H+3% H O(O). Threeelectrode configuration with LSCM/CGO as working electrode and Pt ascounter and reference electrode. The polarization resistances in wet 5%H₂ and wet H₂ were about 0.62 Ωcm⁻² and 0.25 Ωcm⁻², respectively. Theanode polarization in wet H₂ at 925° C. is comparable to that of theconventional Ni—YSZ cermet anode at 1000° C.

EXAMPLE 7 Preparation and Use of Functional Layer

Another possible application of the novel perovskite material of Example1, involves using this material as a thin functional layer,approximately 20-50 microns thick, which is electrochemically active, ontop of a conducting support such as a nickel zirconia cermet or on aporous steel current collector support, and in either case below a dense(<2% porosity) YSZ electrolyte.

In one practical application, a double perovskite LSCM material preparedaccording to Example 1 is used in a composite multi-layered anodestructure with a pure nickel current collector layer, an LSCM outerfunctional layer, and a series of progressively graded Ni-LSCM cermetintermediate layers, as follows:

% Composition Layer Thickness Nickel/LSCM Current Collector 500 μm100/0  Intermediate  10 μm 80/20 Intermediate  10 μm 50/50 Intermediate 10 μm 20/80 Functional  30 μm  0/100

EXAMPLE 8 Preparation and Use of Interconnector

Mix the stoichiometric ratios (as in Example 1) of La₂O₃, SrCO₃, Cr₂O₃and Mn₂O₃ together, add some acetone, ball-mill in a zirconia containerwith zirconia balls for 30 minutes. Fire at 1200° C. for 20 hours twicewith ball-milling with acetone for 30 minutes after each firing, thendry press into 30 mm diameter pellets, and finally fire at 1500° C. for36 hours. This provides a pellet with relative density of approximately94% (i.e. 6% porosity). Typically lanthanum strontium chromite wouldrequire 1600° C. heat treat treatment to achieve similar densification.

FIG. 5 shows schematically an SOFC generator 7 of the the invention forgenerating electricity from fuel gas. The generator 7 comprises atubular SOFC element 8 mounted in a chamber 9 and having an outertubular anode 10 and an inner tubular cathode 11 with an electrolyte 12therebetween, the anode, cathode and electrolyte being of the samematerials as those in the example illustrated in FIG. 2. An air inlettube 13 is provided for introducing pre-heated air 14 into the interior15 of the SOFC element 8, and a fuel gas supply pipe 16 provided forfeeding pre-heated fuel gas 17 into the chamber 9 around the anode 10.Electrical connections 18 are connected to the cathode 11 and anode 10for connection of the generator 7 to an electrical load in use of thegenerator. Exhaust gases 19 comprising spent air, unused fuel, andoxidation products, are vented from the chamber 9 by an exhaust pipe 20,although it is generally preferred that spent air 21 is exhaustedseparately from the unused fuel gas, via a separate exhaust conduit 22.

The invention claimed is:
 1. An anode for a solid oxide fuel cell,wherein at least a part of the anode is a double perovskite oxidematerial having the general formula I:(Ln_(a)X_(b))_(e)(Z¹ _(c)Z² _(d))_(f)O_(g)  (I) wherein Ln is selectedfrom the group consisting of Y, La, a Lanthanide series element, and anycombination thereof and X also represents an element occupying the Asite of a perovskite oxide and is selected from the group consisting ofSr, Ca and Ba, and Z¹ and Z² represent different elements occupying theB site of a perovskite oxide and are selected from the group consistingof Cr, Mn, Mg and Fe, and wherein a has a value from 0 to 1, b has avalue of from 1 to 0, and each of c and d has a value of from 0.25 to0.75, provided that a+b has a value of 1, and c+d, has a value of 1, andwherein e has a value of from 0.8 to 1, wherein f has a value of from0.8 to 1, and g has a value of from 2.5 to 3.2; said material optionallyincluding at least one dopant.
 2. The anode of claim 1 wherein Z¹ and Z²represent Cr and Mn, respectively.
 3. The anode of claim 1 wherein Xrepresents Sr.
 4. The anode of claim 1 wherein said at least one dopantis a B site dopant selected from the group consisting of V, Fe, Cu, Co,Ru, Ni, Pd, Ce, Ti, Nb, Mo and Mg.
 5. The anode of claim 4 wherein the Bsite dopant is present at a level of not more than 20%.
 6. The anode ofclaim 5 wherein the B site dopant is present at a level of from 5 to20%.
 7. The anode of claim 1 wherein in general formula I each of c andd has a value of at least 0.4.
 8. The anode of claim 1 wherein at least30% of the B sites are occupied by a third element Z³.
 9. The anode ofclaim 1 wherein, in general formula I, a has a value of from 0.7 to 0.9.10. The anode of claim 9 wherein, in general formula I, a has a value offrom 0.72 to 0.85.
 11. The anode of claim 1 in which said doubleperovskite oxide material has a porosity of at least 20%.
 12. The anodeof claim 11, in which said double perovskite oxide material has aporosity of from 40 to 50%.
 13. A solid oxide fuel cell having the anodeor functional layer of the anode of claim
 1. 14. An assembly for use ina solid oxide fuel cell said assembly including the anode as defined inclaim
 1. 15. A method of oxidising a fuel in a solid oxide fuel cellcomprising the steps of: a) providing the solid oxide fuel cell havingthe anode as claimed in claim 1; and b) oxidizing said fuel in saidsolid oxide fuel cell.
 16. The method as claimed in claim 15 wherein thesolid oxide fuel cell uses a fuel selected from the group consisting ofhydrogen; a hydrocarbon fuel compound; a hydrocarbon based fuelcompound; a non-hydrocarbon hydride fuel compound, and at least partialreformations thereof.
 17. The anode of claim 1 wherein, in generalformula I, b has a value of from 0.25 to 0.75.